1. Field of the Invention
The present invention relates to the monitoring of current drawn by an ignition coil for a spark ignition engine, and in particular to circuitry and a method for detecting a malfunction in the charging of an ignition coil or its associated drive circuitry.
2. Description of the Related Art
Ignition coil circuitry typically fails either because of a short circuit, for example in leads between a coil driver and the coil itself, or because of an open circuit, for example a break in a winding of the coil.
If either of these conditions happens, the coil will not be charged, and a cylinder will not fire at the desired time. Such faults may be intermittent, and may therefore be difficult to detect using conventional means, for example during routine servicing of a motor vehicle engine. Even when the fault is permanent, it is not possible to tell simply from the misfiring of a cylinder whether the fault is due to an open or a short circuit.
One document concerned with detecting a short circuit is EP 0 502 549-A2, in which a method is disclosed that measures three voltages--the battery supply voltage, and the voltage at either end of a charging coil. Differences between the voltages can then be used to determine if the ignition coil is short-circuited. Such a system is not suitable for detecting more subtle modes of failure, for example those falling short of a complete short or open circuit, and so are not very useful in engine performance diagnosis.
Occasionally, a fault may not be so serious so as to cause misfiring under normal conditions, but may cause misfiring if other engine parameters deviate from normal. For example, high coil resistance may reduce the coil charge, but until the battery voltage falls below a certain level, the charge is still adequate to fire the cylinder. Such a minor fault may become progressively worse, and it would therefore be useful, for example during servicing, to have advance warning of degradation in coil charging.